This invention relates to acoustic devices capable of acoustic action by bending waves and typically (but not exclusively) for use in or as loudspeakers.
Our co-pending earlier parent application Ser No. 08/707,012 includes general teaching as to nature, structure and configuration of acoustic panel members having capability to sustain and propagate input vibrational energy through bending waves in acoustically operative area(s) extending transversely of thickness usually (if not necessarily) to edges of the member(s). Specific teaching includes analyses of various specific panel configurations with or without directional anisotropy of bending stiffness through/across said area(s) so as to have resonant mode vibration components distributed over said area(s) beneficially for acoustic coupling with ambient air; and as to having determinable preferential location(s) within said area(s) for acoustic transducer means, particularly operationally active or moving part(s) thereof effective in relation to acoustic vibrational activity in said area(s) and related signals, usually electrical, corresponding to acoustic content of such vibrational activity. Uses are also envisaged in that earlier parent application for such members as or in xe2x80x9cpassivexe2x80x9d acoustic devices, i.e. without transducer means, such as for reverberation or for acoustic filtering or for acoustically xe2x80x9cvoicingxe2x80x9d a space or room; and as or in xe2x80x9cactivexe2x80x9d acoustic devices with bending wave transducer means, including in a remarkably wide range of loudspeakers as sources of sound when supplied with input signals to be converted to said sound, and also in such as microphones when exposed to sound to be converted into other signals.
Our co-pending US patent application Ser No. 09/246,967, filed Feb. 9, 1999, concerns using features of mechanical impedance in achieving refinements to geometry and/or location(s) of bending wave transducer means for such panel members as or in acoustic devices. The contents of application Ser. Nos. 08/707,012 and 09/246,967 are hereby incorporated herein to any extent that may be useful in or to explaining, understanding or defining the present invention. These applications are collectively referred to herein as the xe2x80x9ctwo prior patents applications.xe2x80x9d
This invention arises particularly in relation to active acoustic devices in the form of loudspeakers using panel members to perform generally as above (and as may be called distributed mode acoustic radiators/resonant panels later herein), but further particularly achieve satisfactory combination of pistonic action with bending wave action. However, more general or wider aspects of invention arise, as will become apparent.
From a first viewpoint, this invention concerns active acoustic devices relying on bending wave action in panel members, particularly providing effective placement(s) for bending wave transducer means different from specific teachings of the two prior patent applications, i.e. other than at location(s) arising from analysis and preference in parent application Ser No. 08/707,012, even including at center(s) of mass and/or geometry rather than off-set therefrom.
From a second viewpoint, this invention concerns acoustic devices relying on bending wave action in panel members, particularly providing effective distributions of resonant mode vibration that may be different from what results from specific teachings and preferences of the two prior patent applications even for the same configurations or geometries.
From a third viewpoint, this invention concerns acoustic devices relying on bending wave action in panel members, particularly providing effective distributions of resonant mode vibration in panel members of different configurations or geometries from what are regarded as inherently favourable in specific teachings and preferences of the two prior patent applications.
It is considered useful to note that effective specific embodiments of this invention utilise panel member(s) intrinsically affording areal distribution of resonant mode vibration components effective for acoustic performance generally comparable or akin to the two prior patent applications, essentially, relying on simple excitement of such intrinsically areally distributed acoustic bending wave action for successful acoustic operation; rather than in any way resembling merely piece-meal provisions for altering intendedly other acoustic action in panel member(s) for which such intrinsic distributed resonant mode action is not even a design requirement indeed, usually where other particular structural etc provisions are made to serve different frequency ranges and/or selectively suppress or specifically produce/superpose vibrations in a panel member that is not intrinsically effective as in two prior patent applications or herein, typically being inherently unsuitable as a matter of geometry and/or location of transducer means.
Effective inventive method and means hereof involve areal distribution of variation in stiffness over at least area(s) of such panel member(s) that are acoustically active in relation to bending wave action and desired acoustic operation. As will become clear herein, such variation can usefully be directly related effectively to displacement of transducer means from location as specifically taught in the two prior patent applications to different locations of this invention, and/or, relative to such patent applications, to rendering unfavourable configurations or geometries of panel members more akin to favourable configurations or geometries for acoustic operation involving areal distribution of resonant modes of vibration consequential to bending wave action, and/or with actual resonant mode distribution that may be at least somewhat different, whether due simply to different areal distribution of bending stiffness hereof or to consequential different location(s) for transducer means, or both.
Specific teaching of parent application Ser No. 08/707,012 extends to panel member(s) having different bending stiffness(es) in different directions across intendedly acoustically active area(s) that may be all or less than all of area(s) of the panel member(s), typically in or resolvable to two coordinate related directions, and substantially constant therealong. In contrast, advantageous panel member(s) of embodiment(s) hereof have variation of bending stiffness(es) along some direction(s) across said area(s) that is/are irresolvable to constancy in normal coordinate or any direction(s).
Areal variation of bending stiffness is, of course, readily achieved by variation of thickness of acoustic panel members, but other possibilities arise, say concerning thickness and/or density and/or tensile strength of skins of sandwich-type structures and/or reinforcements of monolithic structures usually of composite material(s) type.
Whilst available practical analysis may not always allow such investigation as precisely and fully to identify and quantify changes in actual areal distribution of acoustically effective resonant mode vibration for panel member(s) hereofxe2x80x94even where having substantially similar geometry and/or average stiffnesses in relevant directions as for specific isotropic or anisotropic embodiments as per parent application Ser No. 08/707,012xe2x80x94practical resulting performance indicates little if any significant diminishing or degradation in achieved successful acoustic performance involving bending wave action, indeed encourages belief in potential even for improving same. Beneficial effects (on areal distribution of resonant mode vibration), of basically favourable configuration/geometry of the two prior patent applications can, however, be substantially retained to very useful extent and effect in two groups or strands of inventive aspects implementing above one viewpoint.
One group/strand is as already foreshadowed, specifically providing more convenient location(s) for transducer means in acoustically active panel members or areas thereof having configurations or geometries known to be favourable in isotropic or anisotropic implementations of teachings of two prior patent applications, effectively by displacing what are now called xe2x80x9cnaturalxe2x80x9d locations for transducer means (in accordance with these patent applications), to different locations hereof, specifically by either or both of relatively greater and lesser bending stiffnesses to one side and to the other side, respectively, of such natural location(s). Region(s) of greater bending stiffness serve(s) effectively to shift such natural location(s) away from such region(s), typically from said one side towards said other side and region(s) of lesser bending stiffness; region(s) of lesser bending stiffness serving to shift towards own region(s). The other group/strand can be viewed as involving capability only partially to so define same at least notional sub-geometry of larger overall panel member geometry not specifically favourable to good distributed mode acoustic operation as in the two prior patent applications; such sub-geometry being incompletely circumscribed and not necessarily specifically so favourable of itself but the partial definition thereof having significant improving effect on distributed mode acoustic operation, say tending towards a type of configuration or geometry known to include specific favourable ones if not at least approaching such favourable ones; such improving effect being particularly for distributing resonant modes therefor a lower frequencies, but not necessarily (indeed preferentially not) limiting higher frequency bending wave action and resonant mode distribution to such sub-geometry, i.e. allowing such higher frequency resonant mode distribution of vibration past and beyond the partial sub-geometry definition.
As to readily achieving required or desired areal variation of bending stiffness panel member(s) can have at least core layer(s) first made as substantially uniformly two prior patent applications, including sandwich structure(s) having skin layers over core layer(s). Variation(s) of thickness can then be readily imposed to achieve desired areal distribution of stiffness(es). For deformable material(s), such as foam(s), such variation of thickness is achievable by selective compression or crushing to achieve desired contouring, say by controlled heating and application of pressure, typically to any desired profile and feasibly done even after application of any skin layers (depending on stretch capability of such skin layer material). Another possibility is for the member to have localised stiffening or weakening, perhaps preferably graded series thereof. For through-cell or honeycomb materials, e.g. of some suitable reticulated section of its cells extending from skin to skin of an ultimate sandwich structure, or rigidly form-sustaining uncrushable composites, variation of thickness is readily achievable by selective skimming to desired thickness contouring/profiling. None of these possibilities involves necessary change of geometrical center, but skimming rather than crushing inevitably results in change of center of mass. Further alternatives for desired thickness/stiffness variation of as-made core(s) will be discussed, including without change of center of mass as can be important for transducer means combining pistonic and bending wave actions, where pistonic action is manifestly best if centered at coincidence of center of mass and geometric center to avoid differential moments due to mass distribution relative to transducer location(s) and/or to unbalanced air pressure effects.
Center of mass is, of course, readily relocated, typically to geometric center by selective addition of mass(es) to panel member(s) concerned, preferably without unacceptable effects on desired areal distribution of stiffness, e.g. masses also small enough not unacceptably to affect lower frequency bending wave action and effectively decoupled from higher frequency acoustic action(s), say small weight(s) suitably semi-compliantly mounted in hole(s) in the panel also small enough not unacceptably to affect acoustic action(s).
Increasing stiffness in one direction away from or to one side of the xe2x80x98naturalxe2x80x99 location(s) for transducer means location(s) of the two prior patent applications, or decreasing stiffness in a generally opposite direction or to other side, will result in transducer means location(s) hereof generally in said one direction to said one side, which can advantageously be towards geometric center. Such relative increasing/decreasing of stiffness can be complex as to resulting contouring of the panel member concerning, including tapering down increased thickness/stiffness to edge of the panel member and or sloping up decreased thickness/stiffness, say to have a substantially uniform edge thickness of the panel member.
Additionally or alternatively, an inventive aspect of at least the one group/strand is seen in a panel member capable of acoustic bending wave action with a distribution of bending stiffness(es) over its acoustically active area that is in no sense centered coincidentally with center of mass and/or geometrical center of that panel member, though location(s) of acoustic transducer means, whether for bending wave action or for pistonic action or for both, may be substantially so coincident, often and beneficially so.
It is noted at this point that there are two ways in which areal distributions of stiffness(es) over a panel member can be considered or treated as centered, one analogous to how center of mass is usually determined, i.e. as putting first moment of stiffness to zero, thus in a sense corresponding to high stiffness (so herein called xe2x80x9chigh centerxe2x80x9d of stiffness); the other in an inverse manner, putting first moment of the reciprocal of stiffness to zero, thus in another sense corresponding to weakness or low stiffness (so herein called xe2x80x9clow centerxe2x80x9d of stiffness). In panel members with isotropy or anisotropy as specifically analysed in parent application Ser No. 08/707,012, these notional xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d centers of stiffness (so far as meaningful in that context) are actually coincident, further normally also coinciding with center of mass and with geometrical center; but, for a panel member with stiffness distribution as herein, these notional xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d centers of stiffness are characteristically spaced apart and typically further also from center of mass and/or geometric center.
Reverting to effective or notional shifting (by beneficial distributions of stiffness(es) hereof) of practically effective location(s) for bending wave action transducer means (from location(s) afforded by preferred teachings/analyses of the two prior patent applications to different location(s) hereof), such shifting can usefully be viewed as towards said xe2x80x9clow centerxe2x80x9d of stiffness which should thus be along same direction as desired notional shifting, and/or away from said xe2x80x9chigh centerxe2x80x9d of stiffness that may usefully afford at least a structural design reference position for providing variations of bending stiffness(es) in the desired/required corresponding distribution thereof. Variation of bending stiffness outwards from such xe2x80x9clow center(s), to edge(s) of panel member(s) concerned, typically with stiffness(es) increasing to different amounts and/or at different rates in plural directions at least towards xe2x80x9chigh center(s)xe2x80x9d.
Feasible structures of honeycomb cellular cored sandwich type can have desired stiffness distribution by reason of contributions of as-made variant individual cell geometries, and without necessarily substantial effect(s) on distribution and center of mass. Thus, desired areal distributions of stiffness(es) are achievable by variations of cells as to any or all of cell sectional area (if not also shape), cell height (effectively core thickness) and cell wall thickness, including with such degree of progressiveness applied to increase/decrease as may be desired/required. Varying bending stiffness(es) without disturbing distribution of mass is achievable in such context, say by varying cell wall thickness and cell height for nominally same cell area, and/or by varying cell area and/or cell height for same thickness of cell walls, and could, of course, be augmented or otherwise affected by skin variations including varying number and/or nature of ply layers.
Also, it is seen as inventive for panel members hereof to have at least xe2x80x9clowxe2x80x9d centers of stiffness(es) and practically most effective drive location(s) that are identified and typified oppositely in terms of minimum and maximum diversity of transit times to panel edge(s) for notional or actual bending Waves considered as started from xe2x80x9clow centerxe2x80x9d of stiffness and from transducer location(s), respectively.
Reverting to above second general view, panel members with distribution(s) of stiffness(es) as herein (as might perhaps be called xe2x80x9ceccentricxe2x80x9d) can have capability applicable to securing that a said panel of some particular given or desired shape (i.e. configuration or geometry) may exhibit practically effective acoustic bending wave action that was not considered achievable hitherto for that particular shape, at least not according to any prior helpful proposition; including not only for unfavourable shapes related to known favourable shapes, but for shapes not so related but treatable as herein to at least approach what would hitherto be characteristic of some particular favourable shape.
Indeed, this invention extends to capability of some physically realisable areal distribution of bending stiffness(es) of and for even irregularly shaped panel members capable of bending wave acoustic actions to render such action of satisfactorily distributed resonant mode characteristic, and to afford practically effective location(s) for bending wave action transducer means (including by finite element analysis), even irrespective of and without reference to any envisaged or target shape known to be favourable. Such procedures might proceed to at least some extent pragmatically, by trial and error, as to areal stiffness distributions, but can be helped by analysing same using such as Finite Element Analysis at least in terms of affording useful xe2x80x9clowxe2x80x9d and xe2x80x9chighxe2x80x9d centers of stiffness shown herein to have positive (approaching/attracting) and negative (distancing/repelling) location effects on effective location(s) for transducer means within such areal stiffness distribution, whether itself analysable or not.
In practice, useful benefits are seen by way of seeking out constructs and/or transforms by which derivation(s) can be made from what is known to be effective for particular panel member geometries and structures to what may, often will, be effective for a different panel geometry/structure, particularly to indicate structural specification for such different panel geometry as to likely successful areal stiffness distribution and as to transducer drive location(s).
In one approach considered inventive herein, useful attention has been concentrated on transducer location(s), including by way of notionally superposing as a target geometry a desired or given configuration of panel member and a subject geometry of a panel member that is known to be effective and for which detailed analysis is readily done or available, so that desired target transducer location coincides with actual preferentially effective transducer location of the subject geometry. Then, a bending stiffness mapping can be made so that, for any or each of selected constructs relative to now-coincident transducer locations of the target and subject geometries, and over such geometries, so that the known/readily analysed bending stiffness of the subject panel structure can be subject to transformation relative to the target geometry to give substantially the same or similar or scaled comparable stiffness distribution as in the subject geometry and acoustically successful bending wave action in the target geometry. Promising such constructs include lines going from coincident transducer locations to/through edges of the target and subject geometries (say as though representing bending wave transits/traverses). Envisaged related transforms depend on relative lengths of the same construct lines in the target and subject geometries, and a suitable relationship, typically involving the quotient of bending stiffness (B) and mass per unit area (xcexc), i.e. B/xcexc, for proportionality transforms involving the third and/or fourth powers of such line lengths to edges of target and subject geometries. It is preferred, at least as feeling more natural, for a target geometry to be smaller than a related subject geometry, further preferable for superposition to seek to minimise excess of the latter Over the former, including to minimise transform processing. Whilst generally similar types of target and subject shapes may thus be preferred, or favourable subject geometry closest to unfavourable target geometry, it is seen as feasible for the target geometry to differ quite substantially from any recognisable type of known favourable configuration/structure.
It is the case that panels of parent application Ser No. 08/707,012 that are isometric as to areal bending stiffness, and well studied/analysed, are good starting points for subject geometries/structures. Indeed, another construct/transform approach seen as having potential involves seeking to match in the target geometry/structure according to the way that the {now common) transducer location splits bending stiffnesses to each side thereof in the subject geometry/structure. Moreover, similar or related mapping schemes could be used not only as between differing geometry types, but also in the event of wishing or requiring to give to a target geometry of one type such a bending stiffness distribution as to resemble or mimic another type of geometry/configuration, so far as practicable given type of geometry/configuration (e.g. rectangular, elliptical) does have profound influence on actual areal distribution of resonant mode vibration that can be difficult to disturb greatly.
For loudspeaker members capable of both pistonic and bending wave types of action, coincidence of location of bending wave transducer means with center of mass and geometric center is particularly effective in allowing a single transducer device at one location to combine and perform both pistonic drive and bending wave excitation.
It is, however, feasible to use separate transducers one for pistonic-only action at coincident center of mass/geometric center, and another for spaced location conveniently located as herein for bending wave-only action, though mass balancing may then be required by added masses (if not afforded conjointly with requisite distribution of bending stiffness).
A particularly interesting aspect of invention, concerning a single transducer that affords both of pistonic action and spaced bending wave action but at spaced positions, can be used whether spacing is achieved by bending wave transducer location as herein (say to suit convenient transducer configuration) or left as arises without application of above aspects of invention.
Generally, of course, application of this invention may involve distributions of mass with center of mass displaced from geometric center and/or any transducer location, or whatever. Indeed, variation(s) of bending stiffness and/or mass across at least acoustically operative area(s) of panel member(s) can be in many prescribed ways and/or distributions, usually progressively in any particular direction to desired ends different from hitherto, and same will generally represent anisotropy that is asymmetric at least relative to geometric center of mass; and application is seen as in parent application Ser No. 08/707,012.
Practical aspects of invention include a loudspeaker drive unit comprising a chassis, a transducer supported on the chassis, a stiff lightweight panel diaphragm drivingly coupled to the transducer, and a resilient edge suspension surrounding the diaphragm and mounting the diaphragm in the chassis, wherein the transducer is arranged to drive the diaphragm pistonically at relatively low audio frequencies to produce an audio output and to vibrate the diaphragm in bending wave action at higher audio frequencies to cause the diaphragm to resonate to produce an audio output, the arrangement being such that the transducer is coupled to the center of mass and/or geometric center of the diaphragm and the diaphragm has a distribution of bending stiffness including variation such that acoustically effective resonant behaviour of the diaphragm results (at least preferably being centered offset from the center of mass).
The diaphragm may be circular or elliptical in shape and the transducer may be coupled to the geometric center of the diaphragm The diaphragm may comprise a lightweight cellular core sandwiched between opposed skins, and one of the skins may be extended beyond an edge of the diaphragm, with a marginal portion of the extended skin being attached to the resilient suspension.
The transducer may be electromagnetic and may comprise a moving coil mounted on a coil former, the coil former being drivingly connected to the diaphragm. A second resilient suspension may be connected between the coil former and the chassis. One end of the coil former may be connected to the diaphragm, and the said second resilient suspension may be disposed adjacent to the said one end of the coil former, and a third resilient suspension may be connected between the other end of the coil former and the chassis.
The end of the coil former adjacent to the panel diaphragm may be coupled to drive the panel diaphragm substantially at one point. Conical means may be connected between the coil former and the panel diaphragm for this purpose.
The coil former may comprise a compliant section radially offset from a rigid section to drive the diaphragm pistonically and to provide offcenter resonant drive to the diaphragm.
In other aspects the invention provides a loudspeaker comprising a drive unit as described above; and/or is a stiff lightweight panel loudspeaker drive unit diaphragm adapted to be driven pistonically and to be vibrated to resonate, the diaphragm having a center of mass located at its geometric center and a center of stiffness which is offset from its center of mass.